Relatively low-power (e.g., 10 dB above noise floor) signals encoding digital information can be admixed together with composite video signals without being readily evident in television pictures generated from those composite video signals, if suitable restrictions on the digital signal format are observed.
A. L. R. Limberg, C. B. Patel and T. Liu in their U.S. patent application Ser. No. 08/108,311 filed 20 Aug. 1993, entitled APPARATUS FOR PROCESSING MODIFIED NTSC TELEVISION SIGNALS, WITH DIGITAL SIGNALS BURIED THEREWITHIN, and incorporated by reference herein describe phase-shift-keying (PSK) modulation of a subcarrier of the VSB AM carrier that is in quadrature phasing with the VSB AM video carrier of the same frequency. The frequency of their subcarrier is an odd multiple of one-half scan line frequency, and it is phase-shift-keyed in accordance with serial-bit digital data supplied at a symbol rate that is a multiple of scan line frequency. Limberg et alii prefer transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal. Because of frame-averaging effects resulting from the limitations on the speed of the response of the human visual system and on the speed of the decay of electroluminescence of kinescope phosphors, such repetition of data in pairs of frames makes PSK subcarrier accompanying the composite video signal detected from the NTSC television signal less visible in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in a digital-signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal that describes static portions of successive television images. Limberg et alii prefer also repeating the modulation of the digital data in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, providing a basis for using line-comb filtering in the digital-signal receiver to separate PSK subcarrier from the chrominance portion of the composite video signal.
Limberg et alii describe a digital-signal receiver in which the synchronous video detector for quadrature-phase VSB AM video carrier is followed by a cascade connection of a lowpass line-comb filter and a highpass frame-comb filter. The lowpass line-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from chrominance signal portions of the frequency spectrum of an NTSC signal, particularly of an NTSC signal that has been appropriately pre-filtered. The highpass frame-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from motion-free luminance signal portions of the frequency spectrum of an NTSC signal. Limberg et alii teach that the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be viewed as the frequency spectrum of a jamming signal accompanying the PSK signal. Accordingly, the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be discriminated against by synchronous symbol detection.
U.S. patent application Ser. No. 08/141,070, filed 26 Oct. 1993 by J. Yang, entitled APPARATUS FOR PROCESSING NTSC TV SIGNALS HAVING DIGITAL SIGNALS ON QUADRATURE-PHASE VIDEO CARRIER and incorporated herein by reference, describes binary phase-shift-keyed (BPSK) modulation of a suppressed carrier that is the same frequency as a video carrier and is in quadrature phasing therewith. The suppressed carrier is phase-shift-keyed directly, without any subcarrier being used. Yang also advocates transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal, just as Limberg et alii do. Yang advocates the BPSK signals being constrained to about 2 MHz bandwidth, so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering. Yang indicates a preference for passing the data to be transmitted through a pre-line-comb partial-response filter prior to its digital-to-analog conversion to an analog modulating signal for a balanced amplitude modulator. This is done to preserve the information contained therein when line-comb filtering is done in the digital-signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal. Line-comb filtering in the digital-signal receiver converts the partial-response filtered binary digital signal to ternary digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital-signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal. Therefore, multi-level symbol decision circuitry is required to recover bit-serial digital data transmitted by the BPSK from the comb filtering response.
U.S. patent application Ser. No. 08/179,616 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, entitled "PRE-FRAME-COMB" AS WELL AS "PRE-LINE-COMB" PARTIAL-RESPONSE FILTERING OF BPSK BURIED IN A TV SIGNAL and incorporated herein by reference, describes the digital signal transmitter using a pre-frame-comb partial-response filter as well as pre-line-comb partial-response filtering for processing bit-serial data from which BPSK modulating signal is generated for the carrier in quadrature phasing with the video carrier. Line-comb filtering in the digital-signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital-signal receiver converts the partial-response filtered binary digital signal to nine-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal.
U.S. patent application Ser. No. 08/179,588 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, entitled APPARATUS FOR PROCESSING BPSK SIGNALS TRANSMITTED WITH NTSC TV ON QUADRATURE-PHASE VIDEO CARRIER, and incorporated herein by reference, describes BPSK modulating signal for the carrier in quadrature phasing with the video carrier being generated directly from bit-serial data without any pre-comb-filter partial-response filtering. The same patent application describes digital-signal receivers, which use a cascade connection of a highpass frame-comb filter and a highpass line-comb filter after the quadrature video detector to suppress interfering remnant luminance signal, which use plural-level symbol decision circuitry for the comb filter response, and which use post-comb-filter partial-response filtering after the symbol decision circuitry for undoing the data alteration caused by the comb filtering.
Receivers for the Yang system are also described by T. V. Bolger in his U.S. patent application Ser. No. 08/141,071 filed 26 Oct. 1993, entitled RECEIVER WITH OVERSAMPLING ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS WITHIN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
Receivers for the Yang system are also described by J. Yang, T. V. Bolger and A. L. R. Limberg in their U.S. patent application Ser. No. 08/179,586 filed 5 Jan. 1994, entitled RECEIVER WITH SIGMA-DELTA ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS BURIED IN IV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter of sigma-delta type. Preferably, the bit resolution of a basic multiple-bit-resolution flash converter is improved by using a sigma-delta procedure in which only a single bit of the basic multiple-bit-resolution ADC output signal is converted back to analog signal for feedback purposes during each oversampling step, as described by T. C. Leslie and B. Singh in their paper "An Improved Sigma-Delta Modulator Architecture", 1990 IEEE SYMPOSIUM ON CIRCUITS & SYSTEMS, 90 CH 2868-8900000-0372, pp. 372-375, incorporated herein by reference. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
Data can be included in all horizontal scan lines, including all horizontal scan lines in the vertical blanking interval, and data frames started after the vertical sync pulse interval. Alternatively, data frames can begin with the 22nd horizontal scan line of each odd field of composite video signal with data not being transmitted during the 18th through 21st lines of each field of composite video signal. This practice is preferred since it avoids any changes with regard to the 19th lines being used for ghost cancellation reference (GCR) signals, the 20th lines being used for video facsimile transmissions, and the 21st lines being used for closed caption information.
The bandwidths available from the systems described in the patent applications referred to above accommodate the transmission of 5.1-channel Dolby AC-3 audio or MPEG audio.
The inventions described in the patent applications referred to above, like the inventions described herein, are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreements to so assign inventions made within the scope of employment.
By exercise of the superposition theorem for linear systems, the output spectrum of an amplitude-modulation (AM) transmitter having a main carrier amplitude-modulated by a modulating signal including an amplitude-modulated subcarrier can be analogized to the composite spectrum of a first component AM transmitter having a first carrier at main carrier frequency ampitude-modulated by a modulating signal not having the amplitude-modulated subcarrier, a second component AM transmitter having a second carrier above the first carrier in frequency and offset therefrom by a particular phase of the subcarrier frequency, and a third component AM transmitter having a third carrier below the first carrier in frequency and offset therefrom by a particular phase of the subcarrier frequency. The second and third carriers are respectively images of each other on opposite sides of the first carrier to the extent that the third carrier and its modulation sidebands are not suppressed by the vestigial sideband filter. U. S. patent application Ser. No. 08/108,311 advocates a phase relationship between the main carrier and the subcarriers such that the amplitude modulation of the subcarrier is orthogonal to amplitude modulation of the main carrier. The subcarriers modulate the amplitude of an auxiliary carrier of the same frequency as the main carrier and in quadrature phase relationship with the main carrier.
If the subcarrier frequency is high enough, vestigial sideband filtering will eliminate portions of one of the sidebands of the first carrier from the first component AM transmitter and will eliminate the contribution of one of the second and third component AM transmitters from the composite spectrum. Accordingly, that one of the second and third component AM transmitters can be omitted for analytical purposes. In NTSC television the lower sideband of the first carrier is vestigial, so that the contribution of the third component AM transmitter is filtered from the composite spectrum. If the subcarrier frequency is low enough and if the subcarrier sidebands do not extend too far, vestigial sideband filtering will eliminate portions of one of the sidebands of the first carrier from the first component transmitter, but the contributions of both of the second and third component AM transmitters will be retained in the composite spectrum. Analyzing the composite AM spectrum as the superposition of the individual AM spectra in this way establishes a theoretical structure, or analytical model, to support further analysis as underlies the inventions disclosed within this specification.
If one modifies the analytical model to eliminate one of the second and third component AM transmitters altogether, the first component AM transmitter and the remaining one of the second and third component AM transmitters are normally viewed as having individual carriers, the associated frequency spectra of which overlap. However, if the modulation of the carrier of the remaining one of the second and third component AM transmitters was done in such manner as to allow comb filtering to separate that modulation from the modulation of the first component AM transmitter, this separation capability is unaffected by the elimination of one of the second and third component AM transmitters, it is here pointed out. Accordingly, even though an NTSC television signal has buried therewithin a data carrier that does not include image components on each side of the video carrier, the data carrier can be related to the video carrier so as to reduce the visibility of the data in the televised images as reproduced on a television screen, but at the same time permit the frequency spectrum of the data carrier modulation to overlap a substantial portion of the NTSC television signal spectrum. Furthermore, the modulation of the data carrier can be such as to facilitate the separation of the data from video using comb filtering. It is here also pointed out that, if the first component AM transmitter and the remaining one of the second and third component AM transmitters had specific relationships concerning the offset in frequency and phase of their respective carriers that tended to interleave the frequency spectra of their respective modulated carriers in a desirable way, these relationships are affected by the elimination of one of the second and third component AM transmitters only to the extent that during synchronous detection there will be no cancellation of the heterodyne products from the respective carriers of the second and third component AM transmitters. These conclusions flow from consideration of the superposition theorem for linear systems and the separability of the functions describing the modulation results of the first, second and third component AM transmitters.
This means that in a digital-signal receiver the phase of the data carrier, which is suppressed, can still be determined from the phase of the video carrier, which is not suppressed, by regenerating the offset using information from the horizontal scan synchronizing pulses the ghost cancellation reference (GCR) signals or color burst information.
The elimination of one of the second and third component AM transmitters avoids certain problems caused by the interference of their respective modulation spectra.
Where the data carrier is double-sideband (DSB) in nature with both lower-frequency and upper-frequency sidebands having appreciable energy above 750 kHz or so, it is necessary to retain the one of the second and third component AM transmitters having its carrier on the side of the first carrier with a full modulation sideband. The character of the modulation of the data carrier can be QPSK, MPSK, QAM or DSB BPSK. Since there is no interfering image carrier, the carrier frequency of that retained component AM transmitter can be chosen closer to the frequency of the first carrier and the modulation bandwidth increased to utilize the frequency range including the first carrier and its vestigial sideband.
In U.S. patent application Ser. No. 08/108,311 a single-sideband (SSB) binary-phase-shift-keyed (BPSK) data subcarrier was advocated, with the data subcarrier being closer to the video carrier than its SSB BPSK modulation sideband; and the detection of that data subcarrier required suppression of one of the video carrier sidebands, so that the image of the data subcarrier would not interfere with its detection. The elimination of one of the second and third component AM transmitters avoids there being any image of the data carrier that has to be suppressed before the detection of the data carrier can proceed. Retaining the one of the second and third component AM transmitters having its carrier on the side of the first carrier with a full modulation sideband does not change the bandwidth available for the SSB BPSK modulation of the carrier of the retained one of the second and third component AM transmitters, and the bandwidth in the frequency range including the first carrier and its vestigial sideband is not utilized by the SSB BPSK data carrier. Retaining the one of the second and third component AM transmitters having its carrier on the side of the first carrier with a vestigial sideband increases the bandwidth available for the SSB BPSK modulation of the carrier of the retained one of the second and third component AM transmitters, since the bandwidth in the frequency range including the first carrier and its vestigial sideband can be utilized by the SSB BPSK data carrier.
More generally, when an SSB BPSK data carrier is used, its carrier frequency can be chosen closer to one of the boundary frequencies of the transmission channel and its single sideband can be allowed to extend both sides of the video carrier.